IMHO, dealing with fewer computer languages is always better. Especially if YOU are the maintenance programmer. I want to eventually address this same issue for the web site I have recently been assigned to maintain. With multiple languages, one is less likely to become a true master of any one. Why do you think some of these books on Web development technologies have 2, 4, even 8 (!) authors?
So far I've seen no compelling reason to use both VBScipt and JavaScript; but from a code management point of view, plenty of reason not to.
All the books one buys on ASP "teach" utilizing VBScript on the server side. Except for bending to the will of Microsoft, why do this? If I have to pay for all the training, books, tools, etc. necessary for using both languages in my coding shop you better have excellent reasons. So what are they; anyone?
One good reason I use mostly JScript in my server pages is that as a web developer I already know JavaScript. Client-side JavaScript is supported in browsers on many, many platforms so learning it is a good investment, while Basic and VBScript are mostly supported by Microsoft. How many web pages have you seen or even wrote that use client-side VB scripts ? And all books teach VBScript just to go with the flow given by Microsoft. But even the Script Developer guide from MSDN teaches both anyway ...
And then there is also the issue of each language having its advantages and disadvantages. JavaScript is more formal and technical for me, and as a C++ programmer I want that from the language I choose to script with.
Now I can work with both of them anyway. Even though you are right that I do not master any one of them, I think I should learn them both, and then if I want I can specialise into one.